Shaped lignocellulosic-based activated carbon

ABSTRACT

Extruded pellets comprising a majority of activated carbon particles and an organic binder provide improved performance when processed through tumbling equipment while the pellets are in their &#34;green&#34; state, i.e., pellets which are fresh off the extruder and contain activated carbon, binder, and water and have not been subjected to any thermal processing (drying). The tumbling action both smoothes the pellets (by closing any cracks and greatly improving appearance) and increases the density of the pellets, e.g., by increasing particle density and reducing voids between the pellets. Improved performance results from an ability to increase the weight of carbon pellets which can be packed into a fixed volume and thereby increase the volumetric working capacity of the bed for adsorbing/desorbing vapors. Another benefit is to greatly reduce the levels of dust associated with the carbon, both the initial dust and the dust of attrition.

This application is a continuation-in-part of application Ser. No.08/609,632, filed Mar. 12, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an active carbon pellet prepared by extrudingactivated lignocellulosic-based carbon with a binder material. Moreparticularly, the invention relates to an improved active carbon pelletcharacterized by low void volume and low dust attrition.

2. Description of the Prior Art

Granular carbons and carbon pellets are typically used in columns orbeds for gas and vapor systems as well as for processing a number ofliquids. Such carbons have been used in canisters in automobiles throughwhich gasoline tank and carburetor vapors are directed prior to releaseto the environment. To qualify for this application, a carbon mustpossess sufficient mechanical strength to withstand the abrasionincident to continued use.

There generally is a direct correlation between the mechanical strengthof the granular activated carbon product and the mechanical strength ofits precursor raw material. Thus, coal-based active carbon generallyexhibits a high mechanical strength and density; whereas,lignocellulosic-based active carbons, derived from a much "softer"precursor relative to coal, generally exhibit low mechanical strengthsand densities.

Also, gas-adsorbing carbons should be as dense as is consistent withhigh adsorption capacity so as not to require a large space for theadsorber. The development of high adsorption capacity during thermalactivation, however, is accompanied by a loss of mechanical strength anddensity; therefore, some compromise is required in selecting the degreeto which the activation is conducted. So, with lignocellulosicprecursors (or, for lignocellulosic-based active carbons), the problemis compounded.

Several approaches have been taken to address the problem of lowmechanical strength and density of lignocellulosic-based active carbons.In U.S. Pat. No. 3,864,277, Kovach emphasizes the binder additive inteaching the phosphoric acid activation of wood, straw, or low-rankbrown coals in the presence of a carbonaceous binder material such aslignosulfonates and polyvinyl alcohol, followed by forming solidgranular shaped particles from the mixture, and heat-treating at lessthan 650° C. to give a granular product having a ball-pan hardness ofgreater than 85%. Given the teaching of Kovach and employing theknowledge of the relationship of precursor mechanical strength anddensity with those characteristics of the active carbon product,MacDowall (in U.S. Pat. No. 5,162,286) teaches increasinglignocellulosic-based active carbon density by the use of youngcarbonaceous vegetable products high (>30%) in natural binding agent,such as nut shell, fruit stone, almond shell, and coconut shell, asprecursors for treatment with phosphoric acid followed by carbonization.

A third approach, which relates to the present invention, is taught byMcCue et al. in U.S. Pat. No. 4,677,086. To achieve, in a wood-basedactive carbon, the mechanical strength and product density approachingthat achieved with coal-based products, McCue et al. teach extruding anactive wood-based carbon with bentonite clay, followed by calcining theextruded active carbon/clay pellets. This technology has been the basisfor the commercial products NUCHAR® BAX-950 and NUCHAR® BAX-1100marketed by Westvaco Corporation.

In addition to gas column (or, packed bed) requirements for highmechanical strength and high density, it is also desirable to reduce thebed void volume in order to maximize the carbon content of the bed, andsubsequently maximize the adsorptive capacity. This is primarilydetermined by the shape of the granular or pellet carbon. In fact,because of the irregular shape of granular carbon, regularly shapedcarbon pellets are preferred for their better "packing." However, as aresult of uneven cutting of the extrudate to form the pellets, thepellets are usually irregularly shaped, and fissures and cavities oftenappear along the pellet surface. This creates two problems. Theresulting irregularities in shape prevent optimization of bed (orcolumn) packing and detract from maximizing the carbon content for agiven peller volume. In addition, the surface irregularities are oftenremoved from the pellet due to abrasion. These material losses, inaddition to debris caused by cutting the pellets to size, presentanother problem: dust.

Besides having a product which may appear to disintegrate, attrited dustin a packed bed, such as a column or an automotive canister, can fillthe bed voids to create high pressure drops and impede the flow-throughof vapors to be treated. A particular problem in the automotiveapplication is concern that the dust will act to interfere with varioussensing devices connected to the canister to monitor performance.

Typically, dusting due to abrasion, or dust attrition, may be retardedor precluded by spraying a coating on the surface of the pellet.Invariably, this remedy is at the expense of butane working capacity;thereby providing another trade-off for the working life of the activecarbon material.

One solution to this recognized need is described in parent U.S. patentapplication Ser. No. 08/609,632, filed Mar. 12, 1996, and related U.S.patent application Ser. No. 08/613,270, filed Mar. 8, 1996, now U.S.Pat. No. 5,691,270, teaching an improved lignocellulosic-based activatedcarbon pellet of a smoother surface and more uniform shape whichprovides optimal bed packing and which exhibits increased density and isless susceptible to dust attrition and the method of its manufactureusing an inorganic binder material. That invention represents animprovement of that disclosed in U.S. Pat. No. 4,677,086, based onbentonite clay as a binder for activated lignocellulosic-based carbon inan extrusion process. The parent ('632) and related ('270) applicationsteach tumbling the extruded pellet prior to heat treating todry/calcine.

During the development of the improved active carbon pellet, it wassurprisingly discovered that substitution of the inorganic clay binderwith an organic binder material provided a dramatic reduction in bothinitial dust and dust attrition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block flow diagram of the invention process whereintumbling is carried out on the green extrudate, followed by drying andcalcination.

FIG. 2 shows a block flow diagram of the invention process wherebytumbling is carried out on the green extrudate as it is being dried,followed by calcination.

SUMMARY OF THE INVENTION

The object of the invention is achieved in the discovery that extrudedpellets comprising a major portion of activated carbon particles and aminor portion of organic binder provides improved performance whenprocessed through tumbling equipment while the pellets are in their"green" state. Green pellets are those which are fresh off the extruderand contain activated carbon, binder, and water (from 50-70% water, byweight) and have not been subjected to any thermal processing (drying).The tumbling action both smoothes the pellets (by sealing, or otherwiseclosing, any cracks and greatly improving appearance) and increases thedensity of the packed bed e.g., by increasing particle density andreducing voids between the pellets. (Interestingly, debris caused bycutting the pellets to size is assimilated by the tumbling pellets.)Improved performance results from an ability to increase the weight ofcarbon pellets which can be packed into a fixed volume and therebyincrease the volumetric working capacity of the bed foradsorbing/desorbing vapors. Another benefit is to greatly reduce thelevels of dust associated with the carbon, both the initial dust and thedust of attrition.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The process steps for the alternative embodiments of the inventionprocess are set forth in the drawings. The process steps are describedin greater detail in the Examples which follow. Basically, the blend ofactivated lignocellulosic-based carbon, binder material, and water aremixed and then fed through an extrusion device. The generally continuousextrudate is cut at consistent intervals to produce a cylindricalpellet, relatively uniform in length and diameter.

The invention process improvement involves taking these "green" pellets,i.e., soon after they are generated (50-70% moisture content, byweight), and subjecting them to a tumbling process for a period of timesufficient to produce a pellet that, upon subsequent drying, exhibits adust attrition rate of less than 0.75 mg/100 cc/minute. In particular,it has been found that the tumbling step is effective to provide theimproved composition of the invention if it is performed in lieu ofimmediately drying the green pellets in additional equipment.

More specifically, the invention process for preparing an active carbonpellet comprises the steps of:

(a) grinding granular activated lignocellulosic-based carbon to a finepowder;

(b) mixing the activated lignocellulosic-based carbon powder with alesser amount of an organic binder material selected from the groupconsisting of natural and synthetic resins in the presence of water;

(c) extruding the mixture to produce an extrudate which is cut to formgreen pellets are characterized by 50-70% moisture content, by weight;

(d) subjecting the green pellets to a tumbling operation for from about1 to about 30 minutes;

(e) subjecting the pellets to sufficient heat to remove the moisturetherefrom; and

(f) heat treating the dried pellets at from about 700° F. to about 1800°F. in an inert atmosphere for a period from about 0.1 to about 1.0hours, wherein the final active carbon pellets are characterized by adust attrition rate less than 0.75 mg/100 cc/minute, in the absence ofan applied coating on the pellet. This is the process illustrated in theflow diagram of FIG. 1.

In an alternative embodiment of the invention process, as indicated inthe flow diagram of FIG. 2, the tumbling operation may be conducted atsufficient temperatures and for a sufficient time to remove the moisturefrom the pellets, which can then be subjected to the heat treatment.Thus, steps (d) and (e) may be conducted concurrently in a singleoperation.

It is envisioned that the moisture level of the green pellets isimportant in the effectiveness of the tumbling step, and that a criticalmoisture level may exist below which densification and reduction of dustlevels may not occur. As a result, in an additional embodiment of theinvention, the tumbling equipment can also be used to dry the greenpellets, if the drying rate is kept to a level low enough to give asufficient residence time before the critical moisture level is reached.The critical moisture level is in the range of 50-70% water, by weight.A preferred moisture level is 55-65% water, by weight. The mostpreferred moisture level for the tumbling operation of the green pelletsis 58-62% water, by weight.

The lignocellulosic material precursor to the lignocellulosic-basedactive carbon used in the invention process to form the inventioncomposition is selected from the group of lignocellulosic materialsconsisting of wood chips, wood flour, sawdust, coconut shell, nutshells, fruit pits, kernal, olive stone, and almond shell.

The organic binder materials may include natural resins, such as woodrosin, gum rosin, or tall oil rosin based compounds, or syntheticresins, such as styrenic, acrylic, or phenolic based compounds (such asphenol-formaldehyde resin).

Due to the adhesive properties of these organic binders, someimprovement in initial dust and dust attrition may be expected. However,the dramatic improvements achieved by the present invention were whollyunexpected and entirely surprising.

In the Examples to follow, the various analyses were performed inmeasurements determining the benefits of the invention product andprocess:

Apparent Density (AD)--ISO No. 960-050: weight of dry carbon per unitvolume of the carbon bed;

Butane Working Capacity (BWC)--ISO No. 960-080: weight of butane purgedfrom a sample of dried carbon after it had been saturated with butaneper unit volume of the carbon bed;

Dusting Attrition (DA)--ISO No. 960-380: weight of dust attrited from a100 ml sample of carbon per unit time;

Initial Dust (ID)--same as dusting attrition: weight of dust initiallypresent on a 100 ml sample of carbon prior to attrition test;

Actual Pellet Density (APD); weight of dry carbon per unit volume ofentire carbon pellet. Determined using mercury porosimetry;

Base Pellet Density (BPD); weight of dry carbon per unit volume ofcarbon pellet including only pore space less than 0.5 microns equivalentdiameter. Determined using mercury porosimetry;

Bed Void Fraction (BVF): volume of space between carbon pellets per unitvolume of carbon bed. Determined by the equation 1-(AD/APD); and

Pellet Void Fraction (PVF) (pellet interparticle void fraction): volumeof space within a carbon pellet including only pore space greater than0.5 microns equivalent diameter per unit volume of entire carbon pellet.Determined by the equation 1-(APD/BPD).

The invention process and composition are further described in thefollowing examples:

EXAMPLE 1

Ground lignocellulosic-based activated carbon was mixed with novalacresin (a phenolformaldehyde resin), carboxymethyl cellulose (CMC), andwater in a Muller mixer. The dry basis novalac concentration was 10 wt%, and the dry basis CMC concentration was 5 wt %. The mixture wasmulled until it reached a consistency which could be extruded. It wasextruded in a single screw auger extruder through a die plate containing2 mm holes and cut as it exited the die plate into "green" pelletsranging in length from 2-6 mm. The green pellets had a moisture contentof approximately 60 wt % (wet basis). Following extrusion, a portion ofthe green pellets was taken and loaded into a rotating drum in order totumble the pellets. The drum was angled above the horizontal to retainthe pellets and rotated at 30 rpm for 2 hours. Following this, bothportions of the pellets ("tumbled" and "not tumbled") were dried in abatch convection oven. The portion of green pellets which was nottumbled was dried in a batch convection oven. The two batches of driedpellets were heated separately to 1700° F. in a batch indirect-firedrotary furnace for 15 minutes. Following this calcination, they weredischarged and cooled separately under a nitrogen purge prior toanalysis.

The pertinent properties are shown in Table I.

                  TABLE I                                                         ______________________________________                                                                        ID    DA                                              PD       BVF    AD      (mg/100                                                                             (mg/100 cc/                             Batch I.D.                                                                            (g/cc)   (%)    (g/cc)  cc)   min)                                    ______________________________________                                        X-96-0082                                                                             0.639    40.0   0.3833  13.3  0.778                                   Not Tumbled                                                                   X-96-0083                                                                             0.668    36.7   0.4229  3.54  0.256                                   Tumbled                                                                       ______________________________________                                    

Initial dust decreased by 73% and dust attrition decreased by 67%. Also,particle density is improved (by its increase), as is the bed voidfraction (by its decrease).

EXAMPLE 2

In an alternative embodiment of the invention process, groundlignocellulosic-based activated carbon was mixed with novalac resin (aphenol-formaldehyde resin), carboxymethyl cellulose (CMC), and water ina Muller mixer. The dry basis novalac concentration was 10 wt %, and thedry basis CMC concentration was 5 wt %. The mixture was mulled until itreached a consistency which could be extruded. It was extruded in asingle screw auger extruder through a die plate containing 2 mm holesand cut as it exited the die plate into "green" pellets ranging inlength from 2-6 mm. The green pellets had a moisture content ofapproximately 55 wt % (wet basis). Following extrusion, a portion of thegreen pellets was taken and loaded into a rotating drum in order totumble the pellets. The drum was angled above the horizontal to retainthe pellets and rotated at 30 rpm for 2 hours. During this time, thepellets were dried by blowing heated air into the rotating drum. Theportion of green pellets which was not tumbled was dried in a batchconvection oven. The two batches of dried pellets were calcinedseparately to 1700° F. in a batch indirect-fired rotary furnace for 15minutes. Following calcination, they were discharged and cooledseparately under a nitrogen purge prior to analysis.

The pertinent properties are shown in Table II.

                  TABLE II                                                        ______________________________________                                                    AD          ID     DA                                             Batch I.D.  (g/cc)      (mg)   (mg/min)                                       ______________________________________                                        920-A-6     0.432       9.35   0.815                                          Not Tumbled                                                                   920-A-5     0.440       1.10   0.060                                          Tumbled                                                                       ______________________________________                                    

Initial dust decreased by 88% and dust attrition decreased by 92%.

As will be appreciated by those skilled in the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential attributes thereof; and, accordingly, referenceshould be made to the appended claims, rather than to the foregoingspecification, as indicating the scope of the invention.

What is claimed is:
 1. A process for preparing an active carbon pelletcomprising the steps of:(a) grinding granular activatedlignocellulosic-based carbon to a powder; (b) mixing a greater amount ofthe activated lignocellulosic-based carbon powder with a lesser amountof an organic binder material selected from the group consisting ofnatural and synthetic resins in the presence of water; (c) extruding themixture to produce an extrudate which is cut to form green pellets thatare characterized by 50-70% moisture content, by weight; (d) subjectingthe green pellets to a tumbling operation for from about 1 to about 30minutes; (e) subjecting the pellets to sufficient heat for a sufficienttime to remove the moisture therefrom to produce dried pellets; and (f)heat treating the dried pellets at from about 700° F. to about 1800° F.in an inert atmosphere for a period of about 0.1 to about 1.0 hours,wherein the final dried, heat-treated active carbon pellets arecharacterized by a dust attrition rate less than 0.75 mg/100 cc/minute,in the absence of an applied coating on the pellet.
 2. The process ofclaim 1 wherein the binder material is selected from the group ofnatural resins consisting of tall oil rosin, gum rosin, and wood rosin.3. The process of claim 1 wherein the binder material is selected fromthe group of synthetic resins selected from styrenic, phenolic, andacrylic based compounds.
 4. The process of claim 1 wherein the bindermaterial is present in an amount from about 5% to about 30%, by weight.5. The process of claim 1 wherein step (e) is conducted at a temperatureup to 700° F.
 6. The process of claim 1 wherein step (f) is conducted atfrom about 1000° F. to about 1800° F.
 7. The process of claim 1 whereinthe tumbling operation is conducted as a batch process.
 8. The processof claim 1 wherein the tumbling operation is conducted as a continuousprocess.
 9. The process of claim 1 wherein the tumbling operation isconducted up to 15 minutes.
 10. A process for preparing an active carbonpellet comprising the steps of:(a) grinding granular activatedlignocellulosic-based carbon to a fine powder; (b) mixing a greateramount of the activated lignocellulosic-based carbon powder with alesser amount of an organic binder material selected from the groupconsisting of natural and synthetic resins in the presence of water; (c)extruding the mixture to produce an extrudate which is cut to form greenpellets that are characterized by 50-70% moisture content, by weight;(d) subjecting the green pellets to a tumbling operation at a sufficienttemperature and for a sufficient time to remove the moisture therefromto form dried pellets; and (e) heat treating the dried pellets at fromabout 700° F. to about 1800° F. in an inert atmosphere for a period ofabout 0.1 to about 1.0 hours, wherein the final active carbon pelletsare characterized by a dust attrition rate less than 0.75 mg/100cc/minute, in the absence of an applied coating on the pellet.
 11. Theprocess of claim 10 wherein the binder material is selected from thegroup of natural resins consisting of tall oil rosin, gum rosin, andwood rosin.
 12. The process of claim 10 wherein the binder material isselected from the group of synthetic resins selected from styrenic,phenolic, and acrylic based compounds.
 13. The process of claim 10wherein the binder material is present in an amount from about 5% toabout 30%, by weight.
 14. The process of claim 10 wherein step (d) isconducted at from about 60° F. to about 700° F. for from about 1 toabout 300 minutes.
 15. The process of claim 10 wherein step (e) isconducted at from about 1000° F. to about 1800° F.